The present invention relates to dentifrice compositions and especially to clear gel toothpastes.
Dentifrice compositions for promoting the health and appearance of the mouth, and especially of the teeth and gums, are well known. Until recently, these compositions were usually in the form of opaque toothpastes.
Such compositions generally contain agents for cleaning and polishing the teeth. Cleaning, as this word is used in the dentifrice art, refers to the removal of food debris, dental plaque, calculus, tooth surface stains, etc. Polishing refers to the creation of a smooth tooth surface. Typically, these cleaning and polishing agents are abrasives. Abrasives that have been used in opaque dentifrice compositions include calcium carbonate, dicalcium phosphates, tricalcium phosphate, calcium pyrophosphate and analogous water-insoluble magnesium salts.
In addition to abrasives, toothpastes usually contain additional ingredients which serve a variety of purposes. Some examples of such additional ingredients include binders, thickeners, preservatives, sweeteners, colorants, flavors, surfactants, buffers, germicides, antibiotics, astringents and fluorine compounds.
Toothpastes can be dispensed from a variety of containers, such as deformable plastic bottles and aerosol cans. However, they are generally dispensed from collapsible aluminum tubes or collapsible tubes consisting of laminates comprising aluminum. Caps are placed on these tubes in order to protect the product during distribution and to prevent the toothpaste from drying out due to evaporation of water or other volatile materials during use by the consumer
A problem that can arise with such systems is the cementing of the cap to the tube during use of the product by the consumer. This can occur, for example, when toothpaste unintentionally gets onto the threads of the tube and the tube is left undisturbed for a long period of time. This problem is called cap lock and is due to the crystallization of solid material in the toothpaste. In order to prevent cap lock, humectants are added to toothpastes in order to preserve moisture and prevent crystallization.
A related problem is caused by the tendency of some consumers not to replace the cap between uses. The paste in the nozzle of the tube can dry out if precautions are not taken, i.e., if appropriate humectants are not present in the toothpaste in effective amounts.
Sorbitol, glycerine, and mixtures thereof are examples of humectants commonly used in toothpaste compositions. Sorbitol is a sugar alcohol which is generally made by completely hydrolyzing starch and hydrogenating the purified dextrose syrup derived therefrom. The starch may be derived from any convenient source, such as from corn, potatoes, rice or wheat. Most dextrose syrups made in the United States are obtained by hydrolyzing corn starch. In Europe, it is not uncommon to obtain dextrose syrups from potato starch.
Recently, toothpastes in the form of clear gels have been developed; see Pader et al, U.S. Pat. No. 3,538,230 (Lever Brothers Company), Colodney, U.S. Pat. No. 3,906,090 (Colgate-Palmolive Co.) and Wason, U.S. Pat. No. 4,272,509 (J. M. Huber Corporation). Clear gel toothpastes have proved to be very popular with consumers. Undoubtedly, this is due at least in part to their appealing translucent appearance and surprisingly pleasant taste.
The method for assessing translucency in this invention involves use of a standard chart consisting of black symbols varying in size on a white background. This is the RIT Alphanumeric Resolution Test Object, RT 4-74, produced by Graphic Arts Research Center, Rochester Institute of Technology. The ability to discern the symbols clearly through a sample of product of standard thickness is measured. The symbols are assigned numbers from -12 to +13. The higher, more positive the number, the greater the clarity. The method can be applied to measurement of translucency of toothpaste. If even the most prominent symbol cannot be readily defined through the layer of toothpaste, the toothpaste is considered cloudy and not translucent. In the practice of this invention, products with a numerical rating of about -12 or higher are considered translucent.
Difficulties arise in formulating clear gel toothpastes. This is because the abrasives that have generally been used in opaque toothpastes cannot be rendered translucent or transparent. Only materials with the correct combination of abrasive, organoleptic and optical properties are suitable for use as abrasives in clear toothpastes. Certain silica xerogels, precipitates and aluminosilicate salts have been found to have these requisite properties. Silica xerogels and aluminosilicate salts are described in U.S. Pat. Nos. 3,538,230 and 3,906,090, respectively. Silica precipitates are described in U.S. Pat. No. 4,272,509. Silica xerogels are most preferred in this invention, and silica precipitates are also preferred.
It has been demonstrated that translucency or transparency can be obtained when the refractive index of the humectant system is adjusted so as approximately to coincide with the refractive index of the abrasives. For example, it is stated in U.S. Pat. No. 3,906,090 that a sodium aluminosilicate abrasive having a refractive index of about 1.45 to 1.46 forms a clear gel toothpaste when combined with a gel or liquid vehicle having a refractive index of about 1.44 to 1.47. When conventional humectants such as glycerine and sorbitol are employed, the refractive indices of the abrasive and humectant system must be very close to each other for the degree of clarity equal to that defined above to be obtained. A difference in refractive index of, say, more than about 0.02 units at 25.degree. C. would result in clarity less than the minimum defined by the current invention as determined by the method described below.
Sorbitol is often employed as a humectant in clear gel toothpastes. A 70% by weight aqueous solution of sorbitol has a refractive index of 1.46, which is similar to that of the commonly used abrasives for clear or translucent dentifrices. Also, sorbitol has the advantage of being an effective and essentially non-cariogenic sweetener.
Sorbitol, however, is not a sufficiently effective humectant to prevent cap lock completely or to completely inhibit drying out when the silica abrasive dentifrice is exposed to the atmosphere. As a result, glycerine is often admixed with sorbitol. Glycerine's refractive index of 1.47 is similar to that of a 70% solution of sorbitol, which is 1.46. Thus, a mixture of glycerine and 70% sorbitol solution does not have a refractive index substantially different from that of 70% sorbitol alone. Since glycerine adds the humectancy which sorbitol lacks, a mixture of 70% aqueous sorbitol and glycerine is a good humectant for clear gel toothpastes containing either aluminosilicate, precipitated silica or silica xerogel abrasives.
When 70% aqueous sorbitol and/or glycerine comprise the humectant system in a clear gel dentifrice, the index of refraction of the humectant system must closely approximate that of the abrasive in order to establish and maintain the transparency of the dentifrice composition. As a result of this limitation, the amount of water that can be present is severely limited since the presence of water lowers the refractive index of the humectant system while the refractive index of the abrasive remains essentially constant. Therefore, the level of sorbitol and glycerine must remain relatively high. High levels of glycerine are undesirable because of high and rapidly increasing cost. A considerable cost reduction would be effected if a way were found to replace at least some of the glycerine with water in clear gel dentifrice compositions. Likewise, it would be desirable to similarly replace part or all of the sorbitol.
A method for lowering the cost of raw materials which are used as humectants in toothpaste is disclosed in co-assigned U.S. Pat. No. 3,842,167 to Block et al. This patent suggests the replacement of part of the glycerol and/or sorbitol humectant system with aqueous solutions of maltodextrins. Maltodextrins are starch hydrolyzates wherein the saccharides contain substantial amounts of di-, oligo- and polysaccharides. Maltodextrins are normally made by the partial hydrolysis of starch. Such materials are unsuited for use as the sole humectant in toothpastes. As noted in U.S. Pat. No. 3,842,167, these materials could be used as only part of the humectant system. Moreover, maltodextrins are fermentable by cariogenic bacteria, while sorbitol and glycerine are essentially nonfermentable by these bacteria.
Suggestions have been made to add hydrogenated hydrolyzed starch other than sorbitol to opaque dentifrice compositions. Japanese Patent Publication No. 15120/1965 (Towa Kasei Kogyo KK) discloses the addition of a syrup containing 60 to 90% maltitol as a humectant in toothpaste. Maltitol is a sugar substitute obtained by hydrogenating the disaccharide maltose.
U.K. Pat. No. 1,336,944 (Unilever Ltd.) discloses that hydrogenated hydrolyzed starch products described in U.K. Pat. Nos. 1,062,283 and 1,169,538 (Lyckeby Starkelseforadling AB) may be added to opaque toothpaste as sweetening agents. The Lyckeby patents are directed to non-cariogenic sweeteners for candy. These sweeteners, which are marketed under the trade name Lycasin by Roquette Freres, France, are hydrogenation products of mixed mono- and polysaccharides obtained by partially hydrolyzing starch. It is important to note that the requirements for a sweetener in hard candy are different from those of a humectant in toothpaste. In hard candy applications, it is only necessary that the sweetener does not crystallize. In toothpaste applications, on the other hand, it is necessary that the humecant remains non-crystalline and fluid so that the toothpaste will be flowable when subjected to a slight pressure during extrusion from its container.
In U.S. Pat. No. 4,357,314 (ICI Americas, Inc.) hydrogenated maltose containing starch hydrolyzate syrups were disclosed as humectants useful for the preparation of clear transparent gel toothpastes. High maltitol (25-94%) and low sorbitol (5-30%) content characterize these syrups. Maltitol was claimed as the essential component providing humectancy and imparting transparency to the dentifrice gel.
Similar hydrogenated starch hydrolyzates were described in Swiss Pat. No. 620,828 (Gaba AG). Transparent toothpastes were said to be obtainable with a hydrogenated starch hydrolyzate commercially available under the mark "Polysorb 80/55". Again, these hydrolyzates were distinguished by a high maltitol content. The excellent humectancy and clarity retention were specifically ascribed to the presence of maltitol.
As noted in the ICI Americas and Swiss patents, there are cost advantages to using hydrogenated starch hydrolyzates of high maltitol content over glycerol and/or sorbitol. Yet, even these materials are expensive to produce. Manufacture of high maltitol hydrolyzates such as "Polysorb 80/55" and "Lycasin" materials demand partial, very controlled hydrolysis and care in hydrogenation. Special enzymes or bacterial organisms, whose reaction environments must be carefully controlled, are required for the hydrolysis step. Hydrogenation must also be carefully monitored so as to avoid further hydrolysis and not to exceed dextrose equivalent values generally less than 15%. Specifically, special procedures must be used to obtain hydrolyzates with high maltose content. Process specifications for materials such as "Lycasin" are described in British Pat. No. 1,169,538 (Lyckeby Starkelseforadling AG). Bacterial and enzymatic processes for hydrolysis of starch are also reported in German Pat. No. 2,008,853 (Roquette Freres). Discovery of less costly humectants would be an especially welcome advance in this art.
There is a need, therefore, for an effective humectant system suitable for use in a clear gel toothpaste wherein part or all of the glycerine and/or sorbitol can be replaced and wherein there is less of a requirement that the abrasive system and the humectant system have closely matched refractive indices, permitting thereby more water to be present than would otherwise be possible in a clear paste. There is also a need for humectant materials for use in clear gel toothpaste which are less costly to prepare than glycerine, sorbitol or high maltitol content hydrogenated starch hydrolyzate. These substitutes should be sweet, available from a wide variety of starting materials, and not critically dependent on having a specific chemical composition.
A second problem in formulating clear gel dentifrices is that of providing proper texture. Upon extrusion from the tube a dentifrice should retain body. Too frequently, however, the extruded segment slumps. There is poor cohesion. From its perch atop the brush, the paste rapidly spreads forth drooping down into the bristles. Clear gel dentifrices containing humectants with substantial maltitol content suffer from a slumping problem no less severe than equivalent dentifrices made with sorbitol or sorbitol and glycerine.
Relatedly, elegant toothpastes are expected to break sharply upon extrusion from the tube. The ribbon should not string out; it should have shortness of texture. Unlike conventional opaque dentifrices, clear gel dentifrices generally do string out. Inadequate shortness is related to the relatively small amounts (about 25%) of abrasives present in clear gels compared with the 40-50% abrasive content in conventional, opaque dentifrices.
In U.S. Pat. No. 3,689,637, a method is described whereby improved shortness of texture is obtained by incorporating polyethylene glycol of appropriate molecular weight into the dentifrice. Although ameliorative of the problem, polyethylene glycol does not achieve the same shortness of texture as observed with a conventional high-abrasive dentifrice. This solution is
also costly. The instant invention successfully addresses this problem.